The present invention relates to an idling speed (revolutions per minute) adjustment system for an engine in which the quantity of fuel injection during idling is adjusted so as to stabilize the idling speed of the engine.
Recently, some of two-cycle engines etc. adopt an injector for the fuel feed system thereof so as to enhance response, not only in a higher speed range, but also in lower and medium speed ranges, and simultaneously to enhance the purification rate of exhaust gas emission.
In general, in case of setting the quantity of fuel injection of an injector, the speed of an engine and the quantity of intake air detected by an intake air quantity sensor are set as parameters as disclosed in, for example, the official gazette of Japanese Utility Model Registration Application Laid-open No. 169117/1983, or the speed of an engine and the pressure of the lower stream side of a throttle valve detected by a pressure sensor are set as parameters as disclosed in, for example, the official gazette of Japanese Patent Application Laid-open No. 255543/1988.
In addition, a so-called ".alpha.-N control system" is known wherein as disclosed in, for example, the official gazette of Japanese Patent Application Laid-open No. 29039/1988, the quantity of intake air is conjectured from the speed N of an engine and the opening degree .alpha. of a throttle valve detected by a throttle opening-degree sensor.
The .alpha.-N control system can be structurally simplified and is superior in point of cost to the extent that the intake air quantity sensor, the pressure sensor or the like is dispensed with. Therefore, it is oftener adopted for two-cycle engines etc.
Meanwhile, the quantity of intake air passing through the throttle valve in the idle running operation, namely, in a fully-closed throttle condition is small, so that it is liable to disperse every product due to the machining error of a throttle body, etc. Besides, the detection value of a fully-closed throttle region based on the throttle opening-degree sensor is liable to fluctuate due to the assemblage error of this throttle opening-degree sensor, the secular change thereof, etc.
As a result, a difference arises between the quantity of intake air conjectured on the basis of the throttle opening degree .alpha. and the engine speed N and the actual quantity of intake air passing through the throttle valve. Accordingly, the electronic control engine which adopts the .alpha.-N control system has the problem that an appropriate air-fuel ratio cannot be set, so the idling state is rendered unstable.